Ice Sheet
An ice sheet is a mass of glacier ice that covers surrounding terrain and is greater than 50,000 km². The only current ice sheets are in Antarctica and Greenland.
Ice sheet
Ice sheets are bigger than ice shelves or glaciers. Although the surface is cold, the base of an ice sheet is generally warmer, in places it melts and the melt-water lubricates the ice sheet so that it flows more rapidly. This process produces fast-flowing channels in the ice sheet — these are ice streams.
Ice sheets are divided into two types; land based ice sheets, where most of the bottom of the ice sheet lies above sea level and marine-based ice sheets, where most of the bottom of the ice sheet lies below sea level.
The ice sheet in Antarctica is the largest single mass of ice on Earth, but it is classed as two ice sheets to distinguish between the two types of ice sheets. The East Antarctic Ice Sheet (10 million square km) is a land based ice sheet while in contrast, West Antarctic Ice Sheet (2 million square km) is a marine-based ice sheet.
The Antarctic ice sheet is constantly changing, with icebergs breaking off and snow falling. Ice sheets are formed over thousands of years by snow and frost crystals slowly being squashed by more snow falling on top. As these crystals are buried the weight of the crystals above slowly squeezes them together and eventually, they turn into glacial ice.
There has been much speculation that climate change could lead to a collapse of the polar ice sheets. There is little evidence to suggest that the majority of the Antarctic ice sheet has altered significantly in the last 150 years but there is evidence that local changes are underway. Sea ice extent appears to be decreasing, and some ice shelves are breaking up. The most spectacular of these attracted considerable publicity, particularly the collapse of part of Larsen Ice Shelf in 1995.
The Intergovernmental Panel on Climate Change (IPCC) predicted that global mean temperatures will rise by between 1°C and 3.5°C by the year 2100; their "best" estimate is 2.0°C. However, local changes may be quite different to the average. In polar regions precipitation will increase, and Antarctica will probably experience less warming than the Arctic. A few meteorological records from Antarctica have shown more warming than predicted, whilst others have shown no change at all.
While ice shelves are not a direct influence on sea level because they are already floating, it has been argued that the Ross and Ronne-Filchner ice shelves help to stabilise the ice sheet in West Antarctica and so indirectly help to control sea level. Scientists are still unsure if the ice sheet would collapse if these ice shelves retreated. A total loss of the West Antarctic ice sheet would raise sea level by an average of 6m. Fortunately, these ice shelves are a long way south of the Antarctic Peninsula, and the West Antarctic climate is much colder so the local ice sheet is unlikely to be threatened by melting in the next 200 years. The authoritative Intergovernmental Panel on Climate Change (IPCC) says the likelihood of a major sea level rise by the year 2100 due to a collapse of the West Antarctic ice sheet is considered low. For planning purposes, however, the low likelihood must be balanced against the severity of its impact.
Ironically there is an opposing effect that scientists are more confident in predicting. If the southern hemisphere climate warms, warmer air will transport more moisture to Antarctica. This will give more precipitation, and the ice sheet will respond by becoming thicker. So over the next century changes in Antarctica may oppose sea level rise, although they are unlikely to be sufficient to completely counteract the thermal expansion of the oceans and the melting of glaciers outside the Antarctic region.
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